The present invention relates to a processing method of antenna patterns allowing beam width of the antenna patterns to be compressed and a side lobe to be reduced, in an antenna apparatus of monopulse power feed system.
Generally, there are beam width and a side lobe for indices to represent the efficiency of antenna patterns, and the smaller the beam width is or the smaller the side lobe is, the better the performance as antenna patterns is, and it is not only applied to antennas for receiving.
However, there is an opposed relationship between the beam width and the side lobe, and the beam width is inverse proportion to the size (length) of antenna. That is, when the size of antenna is constant, the side lobe will be larger if the beam width is tried to set smaller, and the beam width will be larger if the side lobe is tried to set smaller. When it is allowed to change the size of antenna, the antenna will be larger if the beam width is tried to set smaller, and the beam width will be larger if the antenna is tried to make smaller.
Therefore, for example, in a rader antenna, in the relationship of beam width and a side lobe, resolution capability become worse, identification capability for objects is declined, and many objects might be mistaken for one object, because the beam width is widened when the side lobe is made smaller. Conversely, when the beam width is made smaller, the side lobe is large, therfore, when there is an object to the direction of the side lobe,the wrong judgement that there is an object to the direction for observation might be done, even if there is no object to it. Moreover, in the relationship of the beam width and the size of antenna, when the beam width is made half and the identification level is made twice, the size of antenna will be twice. If the size of it is twice, not only the occupied region of the antenna is larger, but also various bad effects occur such as the increase of the weight of antenna and the enlargement tendency of antenna support structure. Conversely, when the size of the antenna is made half, the beam width is widened twice, and the identification level is worse and half.
Thus, the beam width and the side lobe having an opposite nature, it is impossible to optimize both of them together, so the beam width and the side lobe are made a compromise in a certain degree, considering a distribution for minimizing the beam width under a condition of a certain side lobe, or one for minimizing the side lobe under that of a certain beam width, such as Chebyshev distribution. The beam width and the size of antenna having an opposite nature as described, as practical antennas almost have restriction such as the region occupied by antennas, the beam width is made a compromise in a certain degree in the actual situation.
In order to improve these problems partly, conventionally, it is known that the beam compression method for narrowing the beam width by subtracting a difference signal pattern of each received signal from a sum signal pattern of each received signal from the same two antennas of monopulse power feed system. FIG. 1 is an antenna apparatus such like that performing the beam compression, numerals 101,101 denote antennas of monopulse power feed system with length a and distance d between their centers made equal to the length a, numeral 102 denotes a hybrid circuit (HYB) for forming a sum signal .SIGMA. and a difference signal .DELTA. for each received signal in the two antennas 101,101, numeral 103 denotes detection circuits for detecting the sum signal .SIGMA. and the difference signal .DELTA., and numeral 104 denotes a differential amplifier for outputting an antenna output signal after subtracting the difference signal .DELTA. from the sum signal .SIGMA..
FIG. 2 and FIG. 3 illustrate the result of simulation in the case of using horn antennas with length a about 25.7 times as long as the received wavelength as the antennas 101, 101, and the figures illustrate field patterns (FIG. 2) and power patterns (FIG. 3) normalized on the basis of the value in the direction where the angle is at zero degree, while the aperture surface distribution of the antennas is assumed to be uniform distribution. In these figures for patterns, broken lines show the sum signal .SIGMA. and solid lines show output patterns; from these patterns, it is understood the beam compression is performed.
According to the conventional beam compression method as desribed, the sum signal patterns of the original antenna patterns are given the beam compression, however, large side lobes are generated in output patterns as understood in FIG. 2 and FIG. 3. That is, in a near point where the value of the sum signal .SIGMA. takes zero for the first time, the value of the difference signal a shows a maximum value, then the value subtracted the value of the difference signal .DELTA. from that of the sum signal .SIGMA. is given negative as shown in FIG. 2, but the value b is much larger than the size c of the side lobe of the sum signal pattern. Therefore, in some conventional beam compression methods, a side lobe is enlarged in the point where the difference signal .DELTA. is larger than the sum signal .SIGMA., consequently, although the beam compression can be made, the side lobe will be terribly larger than desired.